Queuing Model The average length of time that a customer waits in line for service is
$W(x, y)=\frac{1}{x-y}, \quad x>y$
where $y$ is the average arrival rate, written as the number of customers per unit of time, and $x$ is the average service rate, written in the same units. Evaluate each of the following.
(a) $W(15,10)$
(b) $W(12,9)$
(c) $W(12,6)$
(d) $W(4,2)$